US media suggest the videos were prepared as “kompromat” – compromising material collected about a politician or public figure in order to create a threat of negative publicity, if needed.

The allegation that Mr Trump was vulnerable to blackmail and was being manipulated financially or otherwise is astonishing, says the BBC’s Paul Wood in Washington.

In a campaign that was unprecedented, this goes to new extremes, our correspondent adds.

How this came to light

The allegations began circulating in political and media circles in recent months.

The BBC understands they are based on memos provided by a former British intelligence officer for an independent organisation opposed to Mr Trump in Washington DC. Sources say the CIA regards them as “credible”.

The original intention was to derail Mr Trump’s candidacy, reports say.

The BBC first saw the documents in October but has been unable to verify the claims included. Several material inaccuracies have been highlighted in them.

However past work by the British operative was considered by US intelligence to be reliable, US media say.

Last week the US intelligence agencies reportedly presented the information – in the form of a two-page synopsis – to the president-elect, to President Barack Obama and to congressional leaders, CNN reported. on Wednesday.

They were published in full by Buzzfeed the same day.

What we know already about Russia hacking claims

In recent weeks, Mr Trump has been under pressure to concur with the findings of US spy agencies who say Russia was behind the hacking of Democratic Party emails during the presidential campaign.

They say the order came from the Kremlin to sway the election for Mr Trump and away from his Democratic opponent, Hillary Clinton.

But so far he has failed to explicitly agree with the conclusions of the intelligence services.

And he has condemned those who oppose good relations with Russia as “fools”.

Russia has denied any involvement in the hacks and accused the US of conducting a witch-hunt.